For the marinade, place all the ingredients, excluding the lamb, for the marinade into a food processor and blend until smooth.
Transfer the marinade to a bowl, add the lamb and leave to marinate for several hours, or overnight, if possible.
For the curry, heat two tablespoons of the oil in a large pan, add the nigella and cumin seeds and stir fry for about 30 seconds, or until the cumin begins to brown.
Add the marinated lamb and cook over a medium heat, stirring frequently, for about 12-14 minutes. Taste to ensure the spices have been cooked - it should have no harsh elements.
Add the water and bring to the boil. Then lower the heat, cover and simmer for 35-40 minutes, or until the lamb is tender. Check on the pot regularly, stirring occasionally and adding more water if it gets too dry.
Halfway through cooking the lamb, heat the remaining oil in a small pan and gently fry the chillies for two minutes, stirring often. Pour into the curry.
Once the lamb is cooked, uncover and stir in the tamarind paste. Taste and adjust the seasoning, add a little extra water if necessary, and stir in the coriander. Serve with naan or rice.